Interoperability and coexistence

Both 11n and 11ac introduced optional features.

Station capabilities and operation

An HT station signals its capabilities using the HT capabilities element,  a VHT station signals its capabilities using both the HT capabilities element and VHT capabilities element . These elements are present in the following management frames:

• Beacon
• Association Request/Response
• Reassociation Request/response
• Probe Request/Response

Beacon transmission

The Beacon is always transmitted in a 20 MHz non-HT format on the primary channel:

• in 2.4 G band, the  DSSS/CCK format is typically used
• in 5 G band, the non-HT OFDM format is typically used

An AP may also transmit a secondary Beacon using an STBC PPDU to extend the rang which Beacon can be detected.

Channel operation

20 MHz BSS operation

An AP indicates 20 MHz BSS operation by setting the Secondary Channel Offset subfield in the HT Operation Information element to 0.

20/40 MHz BSS operation

An HT AP indicates 20/40 MHz BSS operation by setting the Secondary Channel Offset subfield in the HT Operation Information element to a non zero value indicating the offset of the secondary channel relative to the primary channel (+1 or -1).
All 20 MHz transmission would only use the primary channel, the secondary channel should be clear except for the 40 MHz transmission.

The Secondary Channel Offset field

This offset element is constructed as follows:

• Element ID: 1 byte
• Length: 1 byte
• Secondary Channel Offset: 2 bits
• 0
not allowed to transmit in 40 MHz
• 1
the secondary channel is above the primary channel
• 3
the secondary channel is below the primary channel

The Secondary Channel Offset field is set to the value 1 to indicate that ; the value 3 indicates that ; the value 0 indicates that no secondary channel is present, and the value 2 is reserved.